1. Field of the Invention:
The present invention relates to an apparatus and method for treating a fluid, in particular for oxidizing components of an aqueous stream at ambient temperatures. In particular, the invention relates to a system for mixing an oxidizing agent into an aqueous stream followed by selective exposure of the resulting mixture to ultraviolet light of differing wavelengths in an alternating and/or cyclical manner to enhance the effectiveness of the oxidant within the aqueous stream.
2. Description of the Related Art:
Known techniques for oxidizing various contaminants within an aqueous stream at ambient temperatures involve the use of an excess amount of an oxidant to ensure the complete oxidation of the contaminants, or the use of an oxidant along with exposure to ultraviolet light of a single wavelength or wavelength band.
A major problem of simply adding excess oxidant to the stream is the wastage occasioned by such a procedure. In order to ensure complete oxidation of the contaminants, a relatively large amount of oxidant must be added to the stream, not all of which will be used. This requires an abundant source of the oxidant, which may be prohibitive in terms of cost and which may have other disadvantages. While this problem is alleviated to some extent by the known process of irradiating the stream after introduction of the oxidant with a chosen wavelength or wavelength band of ultraviolet (UV) light to photolyze excess oxidant, the problems of wastage of the oxidant have not been satisfactorily addressed. Also, depending on the oxidant and the UV wavelength chosen, undesired byproducts may remain in the aqueous stream after treatment.
The following patents disclose prior art systems for oxidizing contaminants or other constituents of an aqueous stream: U.S. Pat. Nos. 4,141,830, 4,780,287, 4,954,320, and 4,990,311.
U.S. Pat. No. 4,141,830 (Last) discloses an apparatus and method for purifying water. The apparatus consists of a source of UV light which is surrounded by a first annular chamber, with a second annular chamber enclosing both the source and the first chamber. Air is drawn through the first annular chamber and is exposed during its passage through the chamber to the UV light source. The result of the exposure is the generation within the air of minute amounts of ozone (O.sub.3). This treated air is then bubbled through the aqueous stream in the second chamber by admitting it at the upstream end of the second chamber. This mixture flows past the UV source again, and contaminants are oxidized by exposure to both the ozone and the UV light.
U.S. Pat. No. 4,780,287 (Zeff) teaches a method of decomposing volatile halogenated organic compounds. A stream containing such compounds is admitted into a reactor tank having a series of baffles, wherein the stream is exposed to a single source of broad-band UV light. Near the influent port of the reactor, an air/ozone mixture is bubbled into the stream, and oxidation of the compounds is effected by the ozone and the UV light. The resulting volatized compounds are drawn off from the stream and passed into a column of silica gel or quartz chips, the column also being irradiated with the UV light. The resulting gases are passed to a degasser unit which is exposed to a separate UV source to decompose residual ozone. This method involves several different apparatus and does not make complete use of the selected oxidant.
U.S. Pat. No. 4,990,311 (Hirai) teaches a system limited to use with gases wherein within a single chamber the gases are exposed simultaneously to two sources of UV light at different wavelengths. This system also requires that the gases pass through an ozone-decomposing bed.
U.S. Pat. No. 4,954,320 (Birmingham) discloses a somewhat different type of oxidizing system which is, like Hirai, limited to use with gases. Birmingham utilizes a packed bed within which is generated an alternating current electric discharge plasma. The plasma is the primary means of oxidizing any contaminants within the gaseous stream, and it is mentioned that the plasma column can be irradiated with UV light if desired.
The foregoing systems are inefficient, wasteful, and complex. Unlike the present invention, they are not suited to a situation or environment requiring a relatively simple but effective means of decontaminating an aqueous stream while maximizing the usefulness of a limited amount of available oxidant. Existing systems are also specialized to the type of fluid used, and are not capable of processing differing types of fluids with a single apparatus.